Rotary tapered tool holders, commonly referred to as “steep taper” tool holders, are well known in the art. Steep taper tool holders have a male tapered portion extending from a V-flange portion. The V-flange portion has a V-shaped groove to assist the machine tool changer mechanism in gripping the tool. One of the most common steep taper tool holder designs is the Caterpillar V-flange tool holder, generally referred to as a “CV” tool holder. CV tool holders are one of several standards for very similar tool holder designs, all of which have 7/24 tapers (7 inches of diameter change per 24 inches of length). Another common 7/24 tapered tool holder standard is the “BT” tool holder.
The tapered shank portion of the steep taper tool holder is held in a corresponding female tapered portion of a spindle. The tool holder is held in and rotated at high speeds by the spindle. There are generally two types of steep taper tool holders: (1) taper-only contact tool holders, in which only the tapered surface of the tool holder contacts the tapered inside surface of the spindle; and (2) face-taper contact tool holders, wherein the face of the tool holder flange is in contact with the face of the spindle in addition to surface contact between the tapered portion of the tool holder and the spindle. The face-taper contact type tool holder can require a specially designed spindle, wherein the mating face of the spindle is machined more precisely to facilitate operating in contact with the face of the tool holder V-flange portion.
Conventional steep taper tool holders of both types can suffer from certain problems. For example, in a standard steep taper tool holder the taper tolerances for tool holder taper and spindle taper produce a situation wherein the adjacent tapers are in hard contact at the front, but may be out of contact at the rear. When the tool holder is rotated, this divergence of taper angles can produce ‘rocking’ of the tool holder with resulting loss of accuracy and balance. As the spindle is rotated at high speeds, both the spindle taper diameter and the tool holder taper diameter increase under the influence of centrifugal force. However, the spindle taper diameter increases faster than the tool holder taper diameter. Moreover, the diametrical increase is typically not uniform along the length of the spindle taper, but is greatest at the front of the taper. As a result, the spindle taper angle changes, and the tapered surface can even become convex. If the spindle taper were to expand uniformly (maintain the same taper angle), then good fit between spindle and tool holder could be maintained at high speeds via the tool moving axially into the spindle. Unfortunately, because the spindle taper angle changes, the fit between tool holder and spindle degrades at high speeds. The result is two-fold for the standard tool holder. First, since the overall spindle taper diameter increases faster than the tool holder taper diameter, and there is no face contact, the tool holder is drawn into to the spindle (moves axially). Second, because of the taper angle change, the primary contact, which is initially at the front of the taper, moves to the middle or rear of the taper, which results in increased ‘rocking’ of the tool holder in the spindle. The tool holder taper also increases in diameter and changes angle at high speeds, but the amount of change is very small compared to the spindle because the mean diameters of the tool holder are much smaller.
There are also disadvantages encountered with prior art face-taper contact steep taper tool holders. For example, “rocking’ can be greatly reduced. However, as the tool holder is rotated at high speeds, the spindle taper diameter still increases faster than the tool holder taper diameter, although axial positioning is maintained due to the face contact. But, since the tool holder cannot be drawn into the spindle, a radial gap is produced between the tapers, which allows radial motion of the tool holder and results in loss of accuracy and balance.
Another prior art type face-taper contact tool holder uses a tapered sleeve on a shank which moves axially as the rotational speed increases so that the tool holder stays in contact with the spindle. The moveable sleeve can ease tolerancing requirements, but as the tool holder is rotated at high speeds the sleeve moves axially to stay in contact with the spindle. However, the sleeve also increases in diameter due to the centrifugal forces. Therefore, even though the sleeve maintains contact with the spindle, the sleeve can lose contact with the tool holder shank, resulting in a radial gap, thus resulting in unbalance and loss of accuracy.
Another prior art type face-taper contact tool holder uses a sleeve which is split such that it can flex circumferentially and therefore change diameter. The sleeve can thus stay in simultaneous contact with the tool holder shank and the spindle taper as the spindle taper diameter in changing. However, the sleeve still cannot adapt to the changing taper angle, such that contact is still localized at either the front or rear of the taper. Also, friction limits the ability of the sleeve to always maintain solid contact between tool holder and taper, and some ‘slop’ is bound to exist, reducing tool holder stiffness. The split sleeve can also be prone to contamination problems since any material that is present between the sleeve and the tool holder shank will reduce the design's effectiveness, and sealing can be impractical.
Each of the prior art tool holder designs described above, generally in the order listed, can provide an incremental improvement over the previous designs. However, each can also have corresponding increases in mechanical complexity, and all require a face contact to operate. Moreover, although each appear to be effective when at rest, they each have varying limitations at high speeds. In addition, the tight tolerances on the gage diameter is difficult to manufacture and the steep taper angle is not suited for face-taper contact. The spring loaded collets do not provide proper interface forces in the tool holder, thereby limiting rigidity.
Therefore, the prior art face-taper contact tool holders can provide an improvement over the standard tool holder, but they can also have varying limitations at high speeds, increased in mechanical complexity, and all require face contact.
Accordingly, there is a need for an improved steep taper tool holder which can overcome the limitations of the known steep taper tool holders, and reduce or eliminate taper related accuracy and balance problems.